Memory devices using semiconductor elements are broadly classified into two categories: volatile devices which lose stored data when supply of power stops, and non-volatile devices which retain stored data even when power is not supplied.
A typical example of volatile memory devices is a DRAM (a dynamic random access memory). A DRAM stores data in such a manner that a transistor included in a memory element is selected and electric charge is accumulated in a capacitor.
On the basis of the above-described principle, in a DRAM, since charge in a capacitor is lost when data is read, it is necessary to perform writing again so that data is stored again every time data is read. In addition, a transistor included in a memory element has leakage current and charge flows into or out of the capacitor even when the transistor is not selected, whereby data retention period is short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when supply of power stops, an additional memory device using a magnetic material or an optical material is needed in order to hold the data for a longer period.
Another example of volatile memory devices is an SRAM (a static random access memory). An SRAM retains stored data by using a circuit such as a flip-flop and thus does not need refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacitance is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when supply of power stops.
A typical example of non-volatile memory devices is a flash memory. A flash memory has a floating gate between a gate electrode and a channel formation region in a transistor, and stores data by holding charge in the floating gate. Accordingly, a flash memory has advantages in that the data retention period is extremely long (semi-permanent) and refresh operation which is necessary for a volatile memory device is not needed (e.g., see Patent Document 1).
However, because a gate insulating layer included in a memory element deteriorates by tunneling current generated in writing, the memory element stops its function after a predetermined number of writing operations. In order to reduce adverse effects of this problem, a method in which the number of writing operations for memory elements is equalized is employed, for example. However, a complicated peripheral circuit is needed to realize this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. That is, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary for holding charge in the floating gate or removing the charge, and a circuit therefor is required. Further, it takes a relatively long time to hold or remove charge, and it is not easy to perform writing and erasing at higher speed.
As semiconductor thin films applicable to the above-described thin film transistors, silicon-based semiconductor materials have been commonly used, but oxide semiconductors have been attracting attention as alternative materials. Transistors having oxide semiconductors can be manufactured through the same low temperature process through which transistors having amorphous silicon are manufactured, and have higher field-effect mobility than transistors having amorphous silicon. Therefore transistors having oxide semiconductors have been expected to be semiconductor elements that could replace or excel transistors having amorphous silicon.